The present invention relates a process for the preparation of glass fibre reinforced articles from polyester resin.
It is known that the addition of glass fibre to polyester resins may improve the mechanical properties of the resins and particularly their impact strength.
U.S. Pat. No. 3,553,157 discloses the production of articles with thick walls by shaping the resin in the molten state, carried out in the presence of polyfunctional compounds capable of reacting with the resin end groups to lead to an extension of the polymer chain. The polyfunctional compound bringing about an extension of the polymer chain counterbalances the thermal degradation reactions which occur during the shaping process of the article.
It is possible to obtain shaped articles with sufficiently high values of intrinsic viscosity to ensure the necessary mechanical properties of the article.
U.S. Pat. No. 3,553,157 also describes the rein glass fibres reinforced articles.
The fibres are preferably used in a quantity comprised between about 10 and 45% by weight of the polyester resin and the glass fibres.
The polyfunctional compounds most preferred are the polyisocyanates. The use of tetracaboxyilic acid dianhydrides and particularly the use of pyromellitic dianhydride (PMDA) are also described.
As already indicated, the presence of polyfunctional compounds during the shaping phase allows shaped articles with intrinsic viscosity higher than that of the starting polymer to be obtained, and which, in the case of the polyisocyanates, can have an intrinsic viscosity of 3 dl/g or more.
Regarding the use of PMDA, the I.V. highest value is about 0.84 dl/g.
The I.V. increase depends on the PMDA concentration; it reaches a peak and then decreases.
The highest theoretical value of the increase is obtained when the PMDA concentration corresponds to that necessary to react with all the reactive groups of the resin. The viscosity decreases when PMDA is in excess of said value (the maximum of the IV increase obtained when the PMDA reacts with all the reactive end groups of the resin).
Regarding the shaping of articles reinforced with glass fibres, it has been observed that the polymer intrinsic viscosity decreases considerably with respect to the value obtainable in the shaped articles, and is not reinforced with glass fibres. The above situation practically excludes the possibility of obtaining articles reinforced with glass fibres having I.V. values sufficiently high to show the good mechanical properties obtainable with high values of I.V.
It has been unexpectedly found, and this is in contrast with the teaching derived from U.S. Pat. No. 3,553,157, that it is possible to obtain, by melt shaping of polyester resins, articles reinforced with glass fibres having sufficiently high values of I.V. to give the shaped article mechanical properties of high interest.